A new method for estimation of chain lengths of ribonucleic acids is under development. The method involves partial heat degradation of the RNA under study and measurements of the remaining and degraded molecules. From similar measurements on reference RNAs the chain lengths may be deduced. In addition, computer simulation permits computation of the relation between chain length and electrophoretic mobility for use in electrophoretic analysis of polydisperse samples. Heterogeneous nuclear RNA from HeLa cells has been studied by these techniques. The number-average RNA chain is approximately 5,000 nucleotides long; the weight-average is about twice as great.